1. Field of the Invention
This invention relates generally to communication systems, and, more particularly, to wireless communication systems.
2. Description of the Related Art
Conventional wireless communication systems include one or more base stations, which may also be referred to as node-Bs, for providing wireless connectivity to one or more mobile units, which may also be referred to using terms such as user equipment, mobile devices, subscriber equipment, and access terminals. Exemplary mobile units include cellular telephones, personal data assistants, smart phones, text messaging devices, laptop computers, desktop computers, and the like. Each base station may provide wireless connectivity via Radio Frequency (RF) signals to one or more mobile units in a geographical area, or cell, associated with the base station. For example, a base station that operates according to a Universal Mobile Telecommunication System (UMTS) protocol may provide wireless connectivity to one or more mobile units in a cell associated with the base station over a wireless communication link.
The wireless communications link typically includes the ability to deliver information to each of the access terminals using both, or either, multicast and/or unicast transmissions. Typically, a unicast transmission is directed to a single access terminal, whereas a multicast transmission is directed to a plurality of access terminals. Typical multicast applications include the transmission of television signals, radio, audio and video streaming, stock valuations, news summaries, etc. Unicast applications include a typical voice communication session, an Internet session, an instant message session, and the like. In every cell, a single multicast signal is transmitted for all the active users while multiple unicast signals are transmitted individually to each of them.
Superposition coding can be used to multiplex the multicast and unicast signals. That is, the multicast and various unicast signals are superimposed in an orderly fashioned. A simplified version of this technique has recently been proposed and is under discussion for possible adoption in several wireless standards. In this simplified version, the unicast signals are multiplexed in time and/or frequency (i.e., orthogonally rather than superimposed) and only the multicast signal is superimposed thereupon, as is stylistically represented in FIG. 1. On each time/frequency resource, the transmission consists therefore in a superposition of the multicast signal and of a single unicast signal. Arbitrary modulation formats may be used for the multicast and unicast signals.
There are significant differences between the multicast and unicast traffic. For example, the multicast signal is encoded at a fixed rate, established a priori with a goal of ensuring successful decoding in a majority of locations (e.g., 90% to 95%). Furthermore, the multicast signal is often sent synchronously from all the transmission sites in the system and the rate and modulation format is common to all so as to enable constructive combining at the user terminals. The unicast signals, on the other hand, are encoded at variable rates that depend on the channel and interference conditions to the respective user receivers. Such rates may then, in general, be different at each time/frequency resource.
Each user decodes the multicast signal first. If the decoding is successful, it then proceeds to cancel out the contribution of the multicast signal and to further decode the superimposed unicast signal. Since the multicast signal is decoded with interference from the unicast signal, the former must be sufficiently strong with respect to the latter. The total power available for transmission of both the multicast and unicast signal is denoted by PT. The fraction of PT that is allocated to multicasting is denoted as α. Thus, the multicast transmit power is αPT and the available unicast transmit power is (1-α)PT. The standard approach is to use a static α and to account for resulting level of unicast interference when determining the multicast encoding rate.
However, the amount of power required to transmit the multicast signal to the various mobile devices intended to receive the multicast transmission may vary significantly over time. That is the various mobile devices may relocate to a position in which conditions are more conducive such that transmissions need not be as strong. However, given the static α, current systems will continue to transmit at the same power level, unnecessarily limiting the power that may be used for the unicast transmissions.